Freescale Semiconductor TWR-MCF52259-Ethenet - Manual (Page 108)

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The General USB Process
3. As the host enumerates each device it sets
aside bandwidth for the devices that use the
interrupt and isochronous transfer modes
and keeps track of the total bandwidth used.
These two transfer modes can use up to 90%
of the total bandwidth.
Interrupt transfer mode
isochronous transfer mode
bulk transfer mode
4. Once 90% of the available bandwidth is used
up, the host does not allow any other interrupt
or isochronous devices to be enumerated
and the host uses the remaining bandwidth of
at least 10% for control transfer packets and
bulk transfer packets.
The Enumeration Process:
What happens when a device is connected?